My question is, how do I get a diameter symbol for the dimension? Since I'm using a straight line for the frisbee's side view, the dimension automatically just says eg:45, but I want the little circle with the line through it (the diameter symbol) in front of the 45. How do I get this?
However, you should be fine expressing this as a simple line thickness dimension, not the circle with a line through symbol.
That symbol is typically used for curvature to show the distance from the central point to the visible curvature(s), not the thickness or side view of the “frisbee”. Hence the shape of the symbol used to represent the dimension.
You’d use this on the front view, where the frisbee curvature is visible